Sabo lawnmower with Briggs & Stratton engine won't start when warm

Hello,

I have a Sabo 43-vario E lawnmower with a Briggs & Stratton engine (model 124t07 0260 h1).

It starts when cold, but won't start once it's warm—even in the current temperatures. If I spray a little brake cleaner into the carburetor, it starts and runs fine.

As long as I don't turn it off, it runs without issues, and I can mow for extended periods.

I’ve already cleaned the carburetor (using an ultrasonic bath). I think I did a good job, but I’m not sure if there are any "hidden" passages that didn't get fully cleaned out.

I’ve also installed a new ignition coil and spark plug.

What could be the problem?

Many thanks,

Regards,

Raiden

Likely an auto choke issue. Remove the air filter and see if the choke plate opens when hot. You took the vane off to replace the carb and took the housing off to replace the coil. Are you sure you put all back correctly? Is the thermostate lever set up in the correct position? Does the vane move freely? New coils can be larger and scrape the vane preventing it from moving as it should.

I’ve removed the pull-start cord so I can get a better look at the electric starting process. I noticed something interesting while doing so.

The airflow from the flywheel pushes the black plastic arm (yellow arrow)—which controls the carburetor's choke flap—out of the way, meaning the choke effectively stays open as soon as the engine is running. It did start when cold, but I’m not sure if this behavior is correct.

The metal arm (red arrow) attached to the bimetallic strip also moved forward as the engine warmed up; it would have pushed the black plastic arm forward to open the choke (though the choke is already held in the open position by the airflow while running).

When the engine is warm, the bimetallic arm pushes the black plastic arm forward, keeping the choke open. The engine won't start in this position.

However, if I carefully push the black lever backward by hand (against the bimetallic arm) to close the choke, it starts right up—even when warm.

I let the engine run for maybe two minutes; in any case, it was warm enough for the bimetallic arm to have moved fully forward.

Something doesn't seem quite right here.

Don't you think?

2 minutes isn't long enough to warm the engine to restart with the choke open. There was an issue with people with this style choke that have baggers on their mowers, and would start the mower and mow until the bag was full, shutdown the mower to empty the back and then the mower wouldn't restart. Basically the issue that you describe. And from your desciption on how things are working appears to be correct.

Also make sure the choke thermistat is set correctly with the engine cold

ILEngine nailed it for you. You engine looks nice and clean.

Thanks, I'll take a look at it. But I won't get around to it until the weekend.

Also look at the choke arm at the top of the carburetor. These had a tendency to drag on the side of the metal air cleaner backing bracket. They were made with too close of a tolerance.

Pull the air cleaner off with the engine cold and see if the choke flap is fully closed. If it is not, loosen the air cleaner backing plate bolts and see if it then fully closes. If it does, look at the choke lever portion that is plastic to see where it is dragging.

B&S had a shop bulletin on this problem way back when these engines first came out. Basically, the edge of the plastic on the choke lever needs to be sanded just a little to improve the clearance. Maybe just loosening the carburetor/ air cleaner bracket bolts will provide enough clearance, then retighten.

Thanks a lot for these tips as well; I’ll take a look at this over the weekend, too.

What I don't understand, though, is that it starts (with a warm engine) if I push the lever back by hand—against the resistance of the bimetallic lever, which is fully tensioned due to the engine's heat (i.e., closing the choke).

So, the problem probably doesn't lie with the carburetor itself?

Raiden said:

Thanks a lot for these tips as well; I’ll take a look at this over the weekend, too.

What I don't understand, though, is that it starts (with a warm engine) if I push the lever back by hand—against the resistance of the bimetallic lever, which is fully tensioned due to the engine's heat (i.e., closing the choke).

So, the problem probably doesn't lie with the carburetor itself?

Click to expand...

You are now saying if the choke is open and the engine is warm it will not start? however, when you physically close the choke with a warm engine it will start? Go back to what IL Engine stated. Your bimetalic thermostate is not set correctly or not working correctly. If you do not solve the problem there, then I think you need to re-clarify your problem statement.

The choke should not be closed on a hot engine.